Throughout the remainder of this patent application, the term MS refers to a "mobile station" whether operating in a GSM (Global Systems for Mobile communications) type cellular network or in an IS41 (TIA/EIA [Telecommunications Industry Association/Electronic Industries Association] Interim Standard 41) type network. Further, reference to an MS successfully roaming to an IS41 network would be a dual mode (GSM/IS41) MS (an MS that includes transmitters and receivers that can operate in either a GSM or a IS41 environment) which has roamed from its home base GSM provider network and has had its most recent operational contact with an IS41 MSC (Mobile services Switching Center).
In order to allow roaming between "foreign" type networks, a signalling message converter, referred to herein as an IWU (InterWorking/interoperability Unit) operates to interconnect calling and called MSCs. The IWU passes signalling messages back and forth between network entities while converting the format of these messages such that the messages can be properly interpreted by the receiving MSC. A VLR (Visitor Location Register) is a network entity responsible for storing roaming information related to a subscriber. The MSC and VLR entities may or may not be co-located. If they are not co-located, the MSC is responsible for transferring information to the VLR whenever applicable. The IWU is also responsible for keeping VLR and HLR (Home Location Register) data of roaming MSs for the use of connected networks. While the switch in a IS41 network is normally designated as a MTSO (Mobile Telecommunications Switching Office), it is functionally equivalent to an MSC and will be referred to as such for convenience in some parts of the following description.
Although as shown in the accompanying drawings the gateway MSC, the call originating MSC and the home MSC of the roaming MS are the same unit for ease of illustration, these three terms may also refer to three different GSM MSCs. It should also be noted that although the originating caller is shown as a GSM MS and phone to which the call is forwarded is shown as a GSM MS, either or both of these units may be land line phones or phones of completely different type networks from the GSM and IS41 networks shown.
As is known to those skilled in the art, committees were formed to set forth the standards by which cellular systems should work so that equipment from various manufacturers would work together in a given system. However GSM standards are inherently different from the IS41 standards mentioned above. One of these differences is in the operation of "late call forwarding" or the action of the system when a call to a MS cannot be terminated for any of several specific reasons like call collision, no page response of a page request sent to the called MS, and the like. The word terminated as used in the previous sentence refers to the completion of the action of setting up the connection and should not be confused with the completion of the communication that occurs after the connection in finalized. A GSM system expects the destination (call terminating) MSC to handle the call forwarding while an IS41 system expects the originating or home MSC of the MS being called to handle the call forwarding. The standards of IS41, in an attempt to cover the solution to various problems, allowed cellular systems to alternatively have the terminating MSC provide call forwarding when an attempt to terminate the call, by the home location MSC of the MS being called, failed.
As is known by those skilled in the art, call forwarding may occur when a call, to a given mobile station (MS), cannot be terminated (connection completed). The call forwarding can be "early" or "late".
Early call forwarding is defined as occurring when a contact with the MS has failed due to predetermined circumstances occurring while attempting to extend the call to another network.
Late call forwarding is defined as occurring when call forwarding is initiated after the call has been extended to the last known "foreign" public land mobile network location and contact with the MS has failed due to predetermined circumstances.
A NAIG (North American Interest Group) specification committee has attempted to specify interworking, interoperability and interconnection standards to ensure seamless operation of late call forwarding service across GSM type systems and IS41 type systems. By definition herein, seamless service is where MS users are able to roam to other systems from a home system and both make and receive calls in either system in an identical manner as far as the user is concerned. The NAIG approach was to emulate the GSM system by having the terminating MSC (MTSO) handle late call forwarding when a GSM MS was roaming in a IS41 network. The prior art IWU was designed to operate in accordance with the NAIG approach and accordingly late call forwarding was only accomplished in those few instances where the IS41 network supports this functionality in the optional manner of allowing the terminating MSC to provide call forwarding when an attempt to forward the call, by the home location MSC of the MS being called, failed.